1. Field of the Invention
The claimed invention relates generally to antennas for wireless communication; and more particularly, to such antennas configured for transmit and receive function in low frequency applications, including FM radio.
2. Description of the Related Art
The continual search for additional frequency bandwidth and efficiency from antennas in wireless systems points to the need for new approaches. Increasing frequency bandwidth of internal antennas for media applications in cell phones is a prime example. More specifically, internal antennas for FM radio reception in mobile devices are becoming increasingly common. Antenna performance is a key parameter for good reception quality. Mobile handsets are very small compared to wavelengths at FM frequencies; thus the antennas used for these applications on handsets will be electrically small. These electrically small antennas will be narrow band and require low loss matching techniques to preserve efficiency. Multiple electrically small antennas embedded in a small wireless device will tend to couple, thereby degrading performance. The reduced volume allowed for an internal antenna coupled with the fact that the internal FM antennas must not interfere with the main antenna or other ancillary antennas in the handset makes the task of antenna matching across the wide range of frequencies quite difficult.
In order to achieve good efficiency from an internal antenna required to cover the large FM frequency band, one solution is to actively tune the antenna over narrow instantaneous bandwidths. Compared to an antenna structure that covers the whole frequency range without tuning, the tunable antenna greatly improves the antenna radiation efficiency for the same physical volume constraint. Additional active tuned loops can be combined to extend the frequency range to cover wider bandwidths, thereby satisfying a wide range of antenna applications. With the ability to cover multiple octaves, FM and other media applications can be addressed with internal antennas which will provide the required efficiency.
The task of integrating an internal FM antenna into a mobile phone or other mobile device becomes more challenging when a transmit function is required in addition to the receive function. A current application involves the use of an FM transmitter in a portable device that plugs into the headphone jack or other port of a portable audio or video device, such as a portable media player, CD player, or satellite radio system. The sound is then broadcast through the transmitter, and plays through an FM broadcast band frequency. Purposes for an FM transmitter include playing music from a device through a car stereo, or any radio.
The FM-transmitter can be designed to plug into the audio output of audio devices and converts the audio output into an FM radio signal, which can then be picked up by appliances such as car or portable radios. Most devices on the market have a short range of up to 30 feet (9 meters) with any average radio and can broadcast on any FM frequency from 76.0 to 108.0 MHz (or 88.1 to 107.9 in the US), and up to about 75 feet (23 meters) with a very good radio under perfect conditions. Some lower-cost transmitters are designed to operate over a restricted bandwidth which covers 87.7-91.9 MHz band allocated to educational broadcasts in the United States, or a certain other smaller range of frequencies.
FM transmitters are usually battery driven, but some use the cigarette lighter socket in cars, or draw their power from the device itself. A low power design is important for this reason. They are typically used with portable audio devices such as CD or MP3 players, but are also used to broadcast other outputs (such as that from a computer sound card) throughout a home or other building.
It would be beneficial to provide an antenna system that will support transmit and receive using a common antenna element.